1. Field of the Invention
The present invention relates to a method of making hollow bodies such as containers, bottles and the like under pressure from aluminum alloys, type 7475 (as defined by the Aluminum Association) which possess both high resistance and good fracture elongation.
2. Description of the Prior Art
Very light containers, which contain fluids under pressure, and particularly those used for transporting and storing compressed or liquefied gases, have so far been made of type 2001 aluminum alloy (as defined by the Aluminum Association, edition of 1st June 1980). Now, if the use of a product made from an alloy requires the alloy to have excellent resistance to stress corrosion, which is a fundamental requirement for containers under pressure, a very intensive thermal tempering treatment has to be applied to the alloy which results in reduction in fracture elongation of the alloy. In particular, an acceptable hollow container must comply with international regulations in force or in preparation relating to the extent of elongation and the shape of the tear in the container incurred when a hollow body is subjected to a bursting test under hydraulic pressure.
In the making of hollow bodies under pressure, it is necessary that the product obtained have the same mechanical properties (and thus the same lightness) and the same resistance to stress corrosion (and thus the same degree of safety) as hollow body products obtained from alloy 2001. Hollow body products must also satisfy international regulations from the point of view of the extent of elongation and appearance of the tear in the container which a test specimen exhibits during a prescribed bursting test under hydraulic pressure.